Project Summary Spinal cord injury (SCI) is among the most disabling conditions affecting wounded members of the U.S. military. For example, in the Operation Enduring Freedom (2001-present) War in Afghanistan, one in five wounded service members that were evacuated suffered a spinal injury. In fact, the incidence of spinal injuries among combat casualties in the Global War on Terrorism (in both Afghanistan and Iraq) is among the highest in American military medical history. Unfortunately, there has been no effective treatment available for SCI patients. There is, therefore, an urgent need to develop novel repair strategies to mitigate the devastating nature of SCI and clinically translate them to improve quality of life of our veterans with SCI. After SCI, axonal regeneration in a rostrocaudal orientation is crucial for significant functional recovery. Whereas most regeneration studies after SCI have been focused on long supraspinal pathways that project from the brain to the spinal cord [e.g., the corticospinal (CST) and rubrospinal tracts (RST)], regeneration of propriospinal pathways projecting within the spinal cord has been surprisingly understudied. The descending propriospinal tract (DPST) plays an important role in mediating multiple spinal functions, including reflex, posture, and locomotion in normal conditions. Neurons of DPST are mainly located in the intermediate gray matter, receive strong and convergent supraspinal innervations, and project and act on spinal motoneurons and interneurons in multiple cord segments. Among supraspinal pathways, the CST plays a particular role in mediating propriospinal function and its action likely requires an intact propriospinal circuitry. Following SCI, the CST axons fail to regenerate through and beyond the lesion site. Therefore, the innervation of CST axons on DPST neurons and subsequent regeneration of the DPST axons through and beyond the lesion site may provide an alternative pathway or functional relay that transmits supraspinal motor command down to the spinal cord to promote recovery of motor and bladder function. The goal of this application is to establish such a functional relay and examine whether such a relay contributes to functional recovery after SCI. We will test a central hypothesis that successful regeneration and target innervation of DPST axons after SCI provides a structural foundation for a functional relay that mediates supraspinal commands down to the spinal cord and subsequent recovery of motor and bladder function. To test this innovative hypothesis, we will construct a growth-promoting pathway, formed by grafted Schwann cells (SCs) overexpressing glial cell line-derived neurotrophic factor (GDNF), from the lesion site to the caudal host spinal cord in order to promote continued regeneration of DPST axons through and beyond a complete spinal transection at the 10th thoracic level (T10) in adult rats. Using this model, we will determine whether (1) SCI induces degenerative changes of axotomized DPST neurons at their somas and dendrites, and if so, GDNF treatment can reverse such changes; (2) a continuous growth-promoting pathway, formed by grafted SCs overexpressing GDNF (SCs-GDNF), promotes regeneration of DPST axons into the distal host spinal cord, formation of new synapses, and enhancement of functional recovery; (3) supraspinal innervation of CST on DPST is required to direct the functional relay. By completion of this research, new knowledge will be generated concerning whether and how this newly constructed functional relay contributes to recovery of motor function following SCI. If this strategy is proven to be effective, it may be translated to te development of preclinical and clinical trials that may eventually be beneficial for our veteran patients who suffer spinal cord injuries.